EMI shielding gaskets are often used between mating housing components of electronic equipment. The gaskets provide protection against interference from electromagnetic energy, including radio frequency interference (RFI), and more broadly against all bands of interference commonly referred as electromagnetic interference (EMI). The use of EMI/RFI shielding gaskets at the interface between housing components and/or access panels helps to shield the device from sources of electromagnetic interference.
The EMI/RFI shielding gaskets include an electrically conductive element, such as a conductive filler, which is bound by a binding agent and helps to prevent external EMI from interfering with an electronic device having a shielding gasket. Additionally, such EMI shielding gaskets protect other adjacent electronic devices from EMI created by an electronic device having a gasket because the EMI/RFI is prevented from leaking out. The gaskets are used in a wide variety of devices such as cellular phones, notebook computers and other hand-held electronic devices. Given the small size of many of these devices, the housing covers, and the gasket material which is placed between the covers, must be similarly small. This leads to gasket manufacturing difficulties which must be overcome. There are several processes can be used to form gasket material in a desired shape. Examples of such processes include die cutting, compression molding or a process commonly known as form-in-place (FIP).
When an EMI gasket is die cut the gasket configuration is formed from a conductive sheet material which is then cut by a die to the desired shape, such as round, square, etc. Die cutting of an electrically conductive elastomer sheet stock can work in many instances in two plane (i.e. flat) applications. Die cutting an electrically conductive elastomer sheet for use as a gasket is often limited to gaskets having a larger width of gasket material. If the gasket desired is too narrow, often there is not enough material to make the gasket self-supporting. Because gasket material has a narrow width there is often no means to retain the gasket in the desired location prior to package assembly. Because there is no means to retain the gasket, it can fall out of the desired location. The die-cut gasket additionally is difficult to use for many applications, such as an EMI/RFI shielding gaskets for small hand held electronic devices. Die cutting gaskets can also result in a significant waste of sheet stock, thereby increasing the cost of gaskets. Further, the die cutting process exerts a significant amount of force on the sheet stock, and as a result, the sheet stock itself often needs to be fairly stiff and self-supporting. These characteristics are opposite to the desired characteristics of a EMI shielding gasket which should be soft and flexible.
An alternative method of producing an EMI shielding gasket is by compression molding. In this process the gasket is formed by placing uncured elastomeric material, which may contain an electrically conductive filler, into a specifically designed mold which is then subjected to a compressive force. After being formed in the mold, the material is cured to cause the elastomer to assume desired characteristics. This process often has the disadvantage of being relatively slow. Additionally the molding process often creates "flash" or boundary material which is scrap and must be removed prior to the gasket being used. Removing the flash can be a rather slow and labor intensive process, and therefore contributes to an increased cost per part. Further, a mold must be designed for each gasket design. This makes the process expensive and cost inefficient for all but large volume stock items. In addition, the molded gasket cross section may be so narrow that there is often no means to retain the gasket in the desired location prior to assembly. Thus the gasket tends to fall out prior to assembly and is difficult to use in many applications.
The development of a form-in-place EMI shielding gasket is a process where the EMI shielding gasket is extruded directly onto a substrate. Because the gasket is formed directly on the substrate, the waste created using the die cutting or compression molding is avoided. Furthermore, the FIP material usually forms a chemical bond to the substrate to retain the seal in the desired location. Such an FIP process requires a highly precise deposition of a bead of EMI shielding material onto a substrate. In prior art form-in-place deposition there is difficulty when the configuration of the substrate is such that the bead must be discontinued and subsequently continued. The process and apparatus of the prior art can create a "drool" of extruded material which has to be cut off after the bead was formed. This additional step to complete the formation of the gasket can complicate the fabrication of gaskets and can result in increased cost per part. In addition, difficulties have sometimes been encountered in designing form-in-place EMI gasket apparatus to rapidly and efficiently form small cross-section gaskets having desired uniformity with required electrical and mechanical properties.